Rotational overtravel stops are often crucial components of actuators used in environments where not only cost but also reliability, weight, size and simplicity are important. For example, in applications where rotating actuators are used to adjust the flaps of an airplane wing, the reliability of the overtravel stops is crucial in order to prevent possible structural damage and also to prevent the flaps from being adjusted to an aerodynamically unstable position consequent from a failure in the primary control for the actuator. Simplicity and size are also important factors in ensuring the reliability of overtravel stops.
U.S. Pat. Nos. 4,641,737 and 4,721,196 are exemplary of prior art overtravel stops for rotating shafts. The '737 patent applies braking torque to the rotating shaft exclusively by means of the interaction between movable stops and the fixed stops. As a consequence, in applications where substantial braking torque is required, it is necessary that the stops have high torsional load bearing capability in order to provide the necessary braking torque. The '196 patent discloses an overtravel stop for a rotary shaft in which braking torque is applied by the interaction of a movable stop member and a fixed stopped member which has a torsional shock absorber coupled between the fixed stop and ground for transmitting torque from the fixed stop to ground. This design permits the use of the overtravel stop on a high speed shaft of high rigidity while reducing the stopping torque from that which would be present without the shock absorber.
Additionally, overtravel stops for rotary shafts are known in which torsion bars couple the fixed stop to ground to absorb the torque applied to the fixed stop by the rotating stop.
The prior art overtravel stops for rotating shafts having torsion bars or rubber torsional shock absorbers have space and temperature limitations. In high temperature applications rubber will seriously degrade over a period of time and becomes more elastic which limits its torsional load bearing capability. Similarly, torsion bars vary in their spring rate as a function of temperature. Both torsion bars and rubber torsional shock absorbers require an axial space between the fixed stop and ground which can be undesirable in applications where size limitations exist.
Snub valves are hydraulic valves which proportionately control the flow of hydraulic fluid as a function of the position of a valve control member.
Hydraulic control valves also are known which change from an open or closed steady state to the opposite steady state in response to movement of a control member past a reference position. These valves have no intermediate steady state.